Inhumans are an offshoot of the human race. They were biologically crafted to be super powerful weapons. The powers of eachInhuman are ignited when exposed to Terrigen mist the comes from Terrigen crystals. Once exposed an Inhuman begins to mutate in unpredictable ways.The Inhuman race is one of the most powerful groups in the Marvel universe.

First Appearance: Fantastic Four #45(1965)

Origins:
The aliens known as Kree become curious about
humanity when they detect that the ancient and powerful Celestials have
tinkered with human DNA. It is the Celestial tinkering that gives
humans in the Marvel universe the genes that cause mutations. The Kree take a group of humans and further tinker with their DNA creating the
first Inhumans. Due to the Kree tinkering Inhumans are naturally
stronger and longer lived than a peak physical-level human. The
Inhumans learn they can further change themselves by exposing themselves
to a mutagenic gas called the Terrigen Mist. The Terrigen Mist mutates
an individual in unpredictable ways some become monsters, some die, and
some become super powerful.

Factions:
For many years the word Inhuman was almost synonymous with the Inhuman Royal Family. This has changed particularly with the 2014 series Inhuman. The Inhuman series introduced a variety of new Inhuman characters as well as older factions that had been hidden in the universe. The hidden factions included the Ennilux Corporation and the Lor Tribe.

Inhumans in the MCU:
Inhumans are set to have their own movie. The Marvel TV show Agents of SHIELD has begun introducing a variety of Inhuman characters into the Marvel universe. The protagonist Skye is an Inhuman version of the Marvel character Daisy Johnson. Also the villain Raina is an Inhuman. Both characters have interacted with another Inhuman who teleports who is called Gordon.